Dimethyl terephthalate is widely utilized in the manufacture of polyethylene terephthalate, a plastic utilized in plastic beverage containers, Dacron.TM. clothing material and other commercial products. In the manufacturing process, dimethyl terephthalate typically is mixed with ethylene glycol and brought into contact with an acidic catalyst, such as an oxide of antimony. A substantial portion of the dimethyl terephthalate reacts with the ethylene glycol to produce polyethylene terephthalate, methanol, and water. Some intermediate polymers, such as oligomers of dimethyl terephthalate are also produced.
Polyethylene terephthalate and other relatively volatile components are recovered by distillation in a resin tower. As a by-product, the distillation produces a bottoms stream which includes unreacted dimethyl terephthalate, relatively low molecular weight dimethyl terephthalate oligomers, ethylene glycol, and water. Additionally, the bottoms stream often contains a small amount of residual polyethylene terephthalate, traces of the acidic catalyst, and insoluble contaminants. Corrosion products and relatively high molecular weight polymers produced by heat degradation are examples of such insoluble contaminants.
Previously, unreacted dimethyl terephthalate which entered the bottoms stream has been considered unrecoverable. The bottoms stream is not susceptible to conventional filtration because the mixture of dimethyl terephthalate, ethylene glycol, and water quickly fouls conventional filters. Somewhat longer filter runs can be obtained by a filter precoat, such as a coating of diatomaceous earth, on the conventional filters, but such precoats tend to become mixed with the filter cake. Additionally, conventional filtration yields a filter cake of dimethyl terephthalate that is contaminated by catalyst, corrosion products, and thermal degradation polymer products.
Upon leaving the resin tower, the bottoms stream is usually subjected to one or more distillation steps which reclaims ethylene glycol for recycling. Unfortunately, as the level of ethylene glycol in the bottoms stream is reduced, the dimethyl terephthalate and its oligomers become increasingly heat sensitive. Distilling the bottoms stream to about 10% to about 15% ethylene glycol content by volume thermally degrades polymers present in the stream to such an extent that the stream exhibits a viscosity similar to modeling clay.
Consequently, the residual bottoms stream has long been considered worthless. For example, the stream is sometimes incinerated as hazardous waste or placed in landfills.
A need exists for a practical method of separating dimethyl terephthalate and its oligomers from aqueous ethylene glycol streams. Such a recovery process would not only produce valuable dimethyl terephthalate for use in plastic manufacturing, but would also prevent unnecessary filling of increasingly scarce landfills. Additionally, petroleum reserves which are presently consumed to replace the dimethyl terephthalate that is lost could be conserved if such a method were practiced.